In recent years, organic semiconductors are being developed energetically as semiconductors that can be substituted for inorganic semiconductors. Representative examples of an organic semiconductor material include pentacene and polythiophene. In particular, it has been reported that when pentacene is used as a semiconductor in a field effect transistor, a carrier mobility, which is one of the transistor characteristics, achieved by the field effect transistor, can exceed 1 cm2/Vs. In the case where amorphous silicon is used, the mobility achieved is about 1 cm2/Vs. Thus, it is expected that organic semiconductors will replace amorphous silicon. However, in fact, there has been little progress toward such replacement. One major factor hindering the progress toward the replacement is the lifetime of organic semiconductors. Many of the organic semiconductors are susceptible to water and oxygen. Thus, there is a possibility that the semiconductor may be doped with oxygen present in an atmospheric gas so that an OFF current increases to decrease an ON/OFF ratio, resulting in degraded transistor characteristics. Therefore, it is necessary that an organic semiconductor material be stable with respect to oxygen and water so that, in a manufacturing process and a use environment, the doping due to the oxidation can be prevented or minimized. However, developing such an organic semiconductor material requires enormous cost and time.
In conventionally reported organic transistors, an oxide film or insulating resin is provided on an organic semiconductor so as to prevent the entry of oxygen and the like. Although the oxide film is highly resistant to oxygen and water, there has been a problem in that, when forming the oxide film on the semiconductor, a high film forming temperature is required so that the semiconductor may be damaged, degrading its characteristics. On the other hand, the resistance of the insulating resin to oxygen and water is not as high as that of the oxide film. Thus, it is difficult to improve the lifetime of the transistors sufficiently by the use of the insulating resin.
Moreover, the structures of the transistors also pose a problem. For example, in a bottom-gate type transistor, a semiconductor is exposed to the air, so that the contact area of the semiconductor with oxygen is large. In a top-gate type transistor, although the contact area of a semiconductor with oxygen is smaller than that in the bottom-gate type transistor, the semiconductor is vulnerable to oxygen entering from the direction perpendicular to the thickness direction. Furthermore, in a side-gate type transistor in which a semiconductor layer, a source electrode, and a drain electrode are laminated vertically and a gate electrode is formed beside this laminate via an insulating layer, the semiconductor is vulnerable to oxygen entering from the direction perpendicular to the thickness direction as in the case of the top-gate type transistor (see Patent Documents 1 and 2). Therefore, it is possible to improve the lifetime of a transistor by allowing the transistor to have a structure in which oxygen or water cannot enter easily from any directions, thereby protecting an organic semiconductor.    Patent Document 1: JP 2003-110110 A    Patent Document 2: JP 2003-209122 A